I suggest you work to understand his frame of mind and act accordingly. If he wants to make valuable contributions that you believe are needed but just needs his life to be reshuffled to make it happen, you should consider how you can help accommodate those changes.
If he truly is not interested anymore, guide your conversations in that direction without stating your intention to exclude him overtly. You want him to talk until he comes to his own realization that he shouldn't be involved anymore. That's the only way this will occur without his feeling that he was violently pushed out. You have to make this easy, and you have to make it feel like he made the choice, or at least made his own bed, not that you're cutting him off unfairly.